Rating: 4 / 5
Rating 4/5 because the construction was under par. They use tape to form the bivy shape from a single piece of material. The tape job was sloppy with some holes in the bottom. Other than that a great product.
I used a heat sheet bivy as a VB last night in 29* temps and loved it.
Silkweight skin layer, VB and then my Nunatak down quilt rated at 32*.
Normally I would be very cold at 29* as I am a cold sleeper. With the VB I was more than toasty and had to actually sleep a little careless and let some venting occur to maintain the comfort level.
I'd say it adds about 8-10* to my sleep system given that I'm a cold sleeper.
As for the bivy itself, the material is much quiter and stretchy than mylar. As long as no bare skin touches it, adjusting your position is smooth. Bare skin sticks and feels wet. I can also use it as a pack liner which eliminates one more item from my kit and saves an ounce or two. Since you can use clear packing tape or duck tape to patch or tape up your own from two blankets, durability is not a major factor, although it seems to be somewhere between mylar and polycro for strength.
I think I would prefer a pant/shirt combo for ease of movement. I wanted to get my feet out from under the quilt and had to use my hands to get it done. Also, I prefer to have my arms and hands free to make adjustments to the quilt without having to loosen the neck shockcord and lose heat, which I have to do if I am in a liner type VB.